Numerous proposals exist for the gasification of biomass to produce gases containing carbon monoxide, hydrogen and carbon dioxide. For purposes herein, the gases from gasification operations are referred to as synthesis gas (syngas). Anaerobic fermentations of carbon monoxide and hydrogen and carbon dioxide have also been proposed and involve the contact of the substrate gas in a liquid, aqueous menstruum with microorganisms capable of generating oxygenated organic compounds such as ethanol, acetic acid, propanol and n-butanol. The production of these oxygenated organic compounds requires significant amounts of hydrogen and carbon monoxide. For instance, the theoretical equations for the conversion of carbon monoxide and hydrogen to ethanol are:6CO+3H2O→C2H5OH+4CO2 6H2+2CO2→C2H5OH+3H2O.
As can be seen, the conversion of carbon monoxide results in the generation of carbon dioxide. The conversion of hydrogen involves the consumption of hydrogen and carbon dioxide, and this conversion is sometimes referred to as the H2/CO2 conversion. For purposes herein, it is referred to as the hydrogen conversion.
The microorganisms for the anaerobic fermentation of syngas can be adversely affected by components contained in syngas. See, for instance, Xu, et al., The Effects of Syngas Impurities on Syngas Fermentation to Liquid Fuels, Biomass and Bioenergy, 35 (2011), 2690-2696; United States Published Patent Application No. 20110097701; Abubackar, et al., Biological Conversion of Carbon Monoxide: Rich Syngas or Waste Gases to Bioethanol, Biofuels, Bioproducts & Biorefining, 5, (2011), 93-114; and Munasinghe, et al., Biomass-derived Syngas Fermentation into Biofuels: Opportunities and Challenges, Bioresource Technology, 101, (2011), 5013-5022.
Numerous processes have been suggested for the cleanup of syngas for anaerobic fermentation. Often, the processes involve multiple operations to remove different adverse components from the syngas. Xu, et al., state at page 2692:                “Syngas impurities may or may not need to be removed depending upon the effect of the impurity on the biological process and the environment. Selection of commercial technologies suitable for syngas cleanup is mainly based on affordability and the ability to meet end user specifications . . . . Currently, tar cracking methods (including cracking within the gasifier) can effectively convert the heavy and light hydrocarbons to negligible levels. Water quench scrubbers can be employed for removal of ammonia and trace impurities. Accordingly, amine treatment can be utilized for sulfur and CO2 treatment after cooling down the syngas. Zinc oxide beds can also be added for additional sulfur removal down to low levels meeting the requirement for fuel synthesis . . . . For fermentation processes using CO2 as one of the substrates, a different sulfur treatment method should be considered. Alternatively, H2S can be removed from the gasification processes by using regenerable mixed oxide sorbents such as Zinc titanates . . . .”        “Hot catalytic gas conditioning downstream of the gasifier demonstrates more advantages than physical strategies (scrubber+filter). Catalytic strategies provide the possibility to transform the impurities (especially tars and ammonia) into useful gas compounds. By adding cobalt and nickel promoters to Zn—Ti sorbents, both NH3 decomposition and H2S adsorption will occur simultaneously. Most literature has centered on converting tars into useful gases on basic (calcined dolomites) and alumina-supported nickel catalysts at temperatures between 973 and 1173 K. The coupling of a guard bed made from calcined dolomite with a nickel catalytic unit can effectively reduce tar levels to a few ppms . . . .”        
Syngas is typically more expensive than equivalent heat content amounts of fossil fuels. Hence, a desire exists to use syngas efficiently both in the fermentation operation to make higher value products and in conserving the syngas values in any cleanup operation. The financial viability of any conversion process, especially to commodity chemicals such as ethanol and acetic acid, will be dependent upon capital costs as well as the efficiency of conversion of the carbon monoxide and hydrogen to the sought products and the energy costs to effect the conversion.
The cleanup of syngas from biomass is further complicated since biomass is subject to variabilities that can affect gasifier performance and syngas composition. Moreover, a gasifier may from time to time change types of biomass being gasified which may also result in changes in gasifier performance and syngas composition. Thus variabilities in concentrations of components adverse to the fermentation operation, such as hydrogen cyanide, nitric oxide, acetylene and ethylene, occur. Consequently, any cleanup operation would need to have sufficient capacity to handle peak amounts of impurities. Also, the cleanup operation would have to have sufficient turndown capabilities as may be required for cleaner syngas from the gasifier and for startup and non-steady-state operations.
For a biomass to oxygenated organic compound fermentation process to be commercially viable, capital and operating costs must be sufficiently low that it is at least competitive with alternative biomass to oxygenated organic compound processes. For instance, ethanol is currently commercially produced from corn and cane sugar in facilities having name plate capacities of over 100 million gallons per year at sufficiently low costs to be competitive with fossil fuels. Biomass to oxygenated organic compound fermentation processes face even greater challenges due to the multiple major operations required to convert the biomass to syngas, cleanup the syngas sufficiently to be used in an anaerobic fermentation, effect the anaerobic fermentation and then recover a merchantable product.
United States Published Patent Application No. 20100237290 discloses a method for producing a purified syngas from the severe pyrolysis of biomass comprising removing dust and sulfur compounds from the pyrolysis gas, then subjecting the pyrolysis gas to partial oxidation at pressures suitable for conducting a Fischer-Tropsch synthesis and rapidly cooling the gas to a temperature of between 300° C. and 500° C. The patent applicants state that their purified synthesis gas can be used as a feedstock of a Fischer-Tropsch synthesis unit for making liquid fuels and for the synthesis of ammonia, alcohols or dimethyl ether. The patent applicants do not disclose or suggest the use of their purified syngas for anaerobic fermentation nor do they provide any indication of the content in the purified syngas of components that can adversely affect fermentation. Indeed, such components such as benzene and higher aromatics, ethylene and other alkenes, and acetylene and other alkylenes would be desirable in a feedstream to a Fischer-Tropsch synthesis unit.
Processes are sought to convert biomass to oxygenated organic compound at low capital and operating cost but yet provide sufficient robustness that variations in biomass feedstock and gasifier performance can occur without adversely affecting syngas fermentation. Accordingly, the processes need to be characterized by cost-effective syngas cleanup with minimal loss of carbon monoxide and hydrogen yet be able to protect the fermentation from adverse components despite changes in biomass feedstock and changes in gasifier performance.